1. Field of the Invention
The present invention relates generally to automatic flight control systems for aircraft. More particularly, the invention relates to the control surface servomotor control and to the precise measurement of servomotor current, hence servomotor torque, for torque limiting purposes.
2. Description of the Prior Art
As is well known to those skilled in the aircraft autopilot art, safety of flight requirements include provisions for limiting the maximum torque that the autopilot control surface servomotor exerts on the control surface. This requirement is based on the possibility that an autopilot failure could cause a servomotor hardover and therefore if the servomotor had unlimited authority or torque capability, it could cause a severe and dangerous maneuver or threaten the structural integrity of the aircraft. In view of such a possibility, however, remote as it may be, the autopilot designer must demonstrate that servomotor maximum hardover torque is within specified limits before the autopilot may be certified. However, it is also the autopilot designer's goal to provide maximum aircraft controlability under all flight conditions and therefore it is desired to obtain maximum servomotor torque for precise autopilot response and control consistent with the above safety requirements.
In general, many prior art autopilot servomotor controls employ pulse width modulation techniques wherein the magnitude of the total current supplied to the servomotor control windings is a function of the width of current pulses supplied by a current bridge amplifier which in turn is dependent upon the width of voltage pulses supplied by the servoamplifier. A typical pulse width modulation servomotor control system is that disclosed in present assignee's U.S. Pat. No. 3,848,833. As described therein, a power amplifier, driven from a system error amplifier, supplies current pulses to a transistor which in turn controls the bases of the transistors of a power bridge for supplying the current pulses to the control windings of the servomotor. However, in such prior art type systems, the current required to drive the power bridge transistors was not considered in determining maximum servomotor current for torque limiting purposes because it is not possible to calculate current pulse width since it is dependent upon motor load resulting in an error in actual servomotor current measurement by as much as about 7%. Thus, maximum current available for precise autopilot performance had to be reduced by at least this amount to satisfy torque limiting requirements.